Mesoporous hierarchical bismuth tungstate as a highly efficient visible-light-driven photocatalyst

Abstract

The synthesis of flower-like hierarchical bismuth tungstate (Bi₂WO₆) consisting of a mesoporous surface was carried out by a hydrothermal method using the non-ionic surfactant Pluronic F127. The mesoporous and hierarchical surface of the bismuth tungstate was further modified with platinum nanoparticles and the photocatalytic activity was evaluated by studying the removal of rhodamine B under visible light (>420 nm). The effect of synthesis temperature and platinum amount on the photocatalytic activity was investigated and resulting photocatalytic activity of Pt/Bi₂WO₆ was compared with other visible-light-responsive photocatalysts, namely Pt/WO₃, N-doped TiO₂ and Pt/N-doped TiO₂. Photoelectrochemical studies were performed to shed light on the involvement of excited charged carriers in the photooxidation of rhodamine B and a plausible mechanism was proposed based on the photocatalytic and photoelectrochemical behaviour of the catalysts.

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Introduction

Photocatalysis is an attractive and promising future technology for applications in environmental clean-up, clean energy production (H₂ production from water splitting), CO₂ reduction etc., under solar light or other illuminating light source.^1,2 The principles of heterogeneous photocatalytic oxidation processes have been discussed extensively in the literature.^3–5 Briefly, by shining light of energy equal to or greater than the bandgap of a semiconductor, an electron may be promoted from the valence band to the conduction band (e⁻cb) leaving behind an electron vacancy or “hole” in the valence band (h⁺vb). If charge separation is maintained, the electron and hole may migrate to the catalyst's surface where they participate in redox reactions with sorbed species. Specifically, a h⁺vb may react with surface-bound H₂O or OH⁻ to produce a hydroxyl radical (OH˙), and an e⁻cb is scavenged by oxygen to produce a superoxide radical anion (O₂⁻˙). These extremely reactive species are primarily responsible for the photo-degradation of organic pollutants.^3–5 In spite of the presence of these efficacious radicals, efficiency of the photocatalytic process remains low and development of such photocatalysts that could produce a surpassing number of these radicals with extended lifetimes continues to be a challenge. Among the widely researched photocatalysts, titania is the most studied and proficient photocatalyst but the major drawback associated with TiO₂ is that it can only harness energy limited to the UV region due to its rather large bandgap of 3.2 eV. Attempts have been made to induce visible light photocatalytic activity in TiO₂ by doping with metals and non-metals.^6–9 Although metal or anion doping seems to be the most effective strategy for narrowing the bandgap in order to enhance visible light absorption, impurity sites act as carrier recombination centers where photoexcited electrons and holes are annihilated by combining with each other before participating in chemical reactions on the surface of the photocatalyst.¹⁰ Thus, even though activity was generated under visible light in certain cases, a dramatic improvement of the overall photocatalytic efficiency was not achieved by means of impurity doping. On the other hand, metal chalcogenides such as CdS, CdSe, etc. have been widely studied as visible-light-active photocatalysts.^11–15 However, these chalcogenides are unstable, i.e., they suffer from photo-corrosion phenomena.^16–18 The development of catalysts, therefore, that are active under visible light and stable under photocatalytic conditions remains a challenge. For this reason, many other materials have been extensively explored and the search continues in the hope of finding desirable band structures and efficient activity under visible light irradiation.

Recently, owing to the layered structure and unique properties, semiconductor photocatalysts of the Aurivillius oxides Bi₂A_n−1B_nO_3n+3 (A = Ca, Sr, Ba, Pb, Na, K, and B = Ti, Nb, Ta, Mo, W, Fe) have drawn particular attention.¹⁹ Among these Aurivillius oxides, Bi₂WO₆ has been the subject of many photocatalytic studies including water splitting and photodegradation of organic pollutants under visible light irradiation.^20–26 Moreover, the stability of Bi₂WO₆ has been demonstrated to be excellent under photocatalytic conditions.^27–29

Since the catalytic reactions take place on the surface of the catalysts, surface engineering is critical in designing active materials. Among the surface engineering strategies, creating roughness or porosity on the surface seems promising, in particular because it is likely to provide sufficient surface area for the adsorption of reactants and faster migration or diffusion of the parent as well as intermediate products, thereby enhancing the overall efficiency of the process. Moreover, materials having a hierarchical structure are of particular interest.^30–32 The aim of the study presented here was to synthesize hierarchical Bi₂WO₆ in mesoporous form, modify the mesoporous surface with Pt, and then test & compare the photocatalytic activity with other active visible-light-driven semiconductor photocatalysts. Moreover, the study of the photoelectrochemical behavior and its correlation with photocatalytic activity was also included in the scope of this study.

Experimental

Synthesis of catalysts

In a typical synthesis, 0.5 g of surfactant was completely dissolved in 20 ml of water acidified with nitric acid (pH ∼ 1). Bismuth nitrate (0.08 M) was added and the solution was stirred until a clear solution was obtained. In addition, an aqueous solution of sodium tungstate (0.04 M, 20 ml water) was prepared separately. The solution of sodium tungstate was added dropwise to the mixture of bismuth nitrate with vigorous stirring at room temperature. As a result, a white colloidal solution was obtained, which remained under stirring for 2 h. Then, the resulting white colloidal solution was transferred into a Teflon vessel and heated at different temperatures for 20 h under autogenous pressure. After cooling, the product was collected, thoroughly washed with water and ethanol to remove any residual surfactant and dried at 110 °C under vacuum overnight. Non-mesoporous samples were prepared following the aforementioned procedure excluding the addition of surfactant.

Synthesis of N-doped TiO₂ was carried out by annealing a TiO₂ sample under NH₃ flow following the procedure reported elsewhere.³³

Photodeposition of Pt nanoparticulates

The deposition of platinum onto the surface of the photocatalysts was performed using a photodeposition method in an immersion well photochemical reactor. A detailed schematic of the photo-reactor has been provided in our previous work.³⁴ Briefly, 130 ml of water was added to the reaction vessel and the required amount of metal salt (H₂PtCl₆) and photocatalyst was added. The suspension was stirred and purged with high purity argon gas for at least 30 min to remove the dissolved oxygen. Methanol (10 vol%) was added as an electron donor. Irradiation was carried out using a 230 W tungsten-halogen lamp (OSRAM) for 6 h. After irradiation, the Pt loaded catalyst was washed with water and ethanol and separated through centrifugation and dried at 110 °C under vacuum overnight.

Characterization

Characterization was carried out by employing High-angle Annular Dark-field Transmission Electron Microscopy (HAAD-TEM), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), BET surface area analysis and Diffuse Reflectance Spectroscopy (DRS).

Evaluation of photocatalytic activity

The photocatalytic tests were performed in a photocell equipped with a quartz window. Rhodamine B (RhB) dye was selected as the model pollutant to study the photocatalytic performance of the synthesized samples. For irradiation experiments, 100 ml of dye solution with the desired concentration was taken into the photocell and the required amount of photocatalyst (1 g l⁻¹) was added. Irradiation was carried out by a 300 W xenon lamp using a cut-off filter to obtain a wavelength of >420 nm. A light guide, kept at a 10 cm distance from the photocell, was used to illuminate 1 cm² area of the photocatalyst's surface. Samples (∼3 ml) were taken at regular time intervals and catalyst particles were removed by filtration using a 0.45 μm filter before spectrophotometric analysis. The decomposition (decrease in absorption intensity vs. irradiation time) of the dye was monitored by measuring the change in absorbance at ∼553 nm (λ_max) on a UV-Vis spectrophotometer and % removal was calculated using the formula below.
where C₀ = initial concentration, and C_t = change in concentration with irradiation time.

Evaluation of photoelectrochemical activity

Photoelectrochemical activity was studied using a three electrode quartz photoelectrochemical cell connected to a potentiostat. Silver/silver chloride (Ag/AgCl) and platinum (Pt) electrodes were used as the reference and counter electrodes, respectively. A homogeneous suspension was prepared through sonication of a mixture of photocatalyst, ethanol and Nafion. The resulting suspension was deposited on an Indium Tin Oxide (ITO) substrate, dried, and used as the working electrode. Sodium sulphate was used as the electrolyte. A similar irradiation source and wavelength (>420 nm), as employed for the photocatalytic studies, were applied. High purity Ar gas was used to remove the dissolved O₂ from the electrolyte solution as needed.

Results and discussion

The crystal structure and degree of crystallinity of bismuth tungstate were determined by XRD, and the obtained diffraction patterns are presented in Fig. 1(A). The spectra indicated a well-crystalline russellite structure of bismuth tungstate, which is characterized by an orthorhombic structure made up of WO₆ octahedra layers sandwiched between the layers of bismuth and oxygen. As shown, the patterns of mesoporous and non-mesoporous bismuth tungstates were found to be identical. Any impurity in the form of Bi₂O₃ was not detected in any of the samples indicating the complete reaction of bismuth with WO₆. In order to ensure that the mesoporous sample was free from carbonaceous entities or surfactant which may interfere in the photocatalytic process, FTIR analysis of samples was carried out, as illustrated in Fig. 1(B). It can be seen that both the spectra are identical, presenting onset bands of Bi₂WO₆,^21,35 and indicating the complete removal of surfactant or its derivatives. Since hydrothermal synthesis temperatures may have a significant effect on the phase, shape and morphology, BET surface area and other properties of the samples which in turn regulate the photocatalytic properties of photocatalysts, this effect was studied whereby samples were synthesized at 130, 150, 170 and 190 °C and the changes in phase and morphology were followed by XRD and FESEM analysis. As shown in Fig. S1,† no obvious difference was observed in the XRD patterns of samples synthesized at different temperatures except for an enhancement in the crystallization with increasing synthesis temperature. However, the shape and morphology were found to be affected by higher synthesis temperature. Representative images of the mesoporous sample synthesized at 170 °C are illustrated in Fig. 2(A) and (B). The images show the formation of a self-assembled flower-like morphology with hierarchical architecture. The diameter and thickness of the spheres were measured to be ∼2–4 μm and <1 μm, respectively. The hierarchical architecture was found to be composed of rod-like crystallites, as presented in Fig. 2(B). No morphological difference was observed between the mesoporous and non-mesoporous samples. Microscopic observations revealed that the hierarchical structure was formed at 130 °C and remained unaffected by the synthesis temperature up to 170 °C. However, when the temperature was increased to 190 °C, the hierarchy was slightly lost, as shown in Fig. S2,† presumably due to the diffusion of particles at higher temperature. The shape & size of the Pt nanoparticles deposited on the Bi₂WO₆ surface was determined by HAAD-TEM and the obtained image is depicted in Fig. 2(C). The shape of the Pt was found to be spherical and the size was estimated to be ∼2 nm. Furthermore, compositional analysis of the Pt/mesoporous Bi₂WO₆ nanocomposite, as shown in Fig. 2(D), indicated the presence of Bi, W, O and Pt, and confirmed that the atomic ratio of Bi and W was close to the stoichiometric ratio (2 : 1) in Bi₂WO₆. A schematic showing the possible morphological evolution of Bi₂WO₆ into a flower-like hierarchical structure is presented in Scheme 1. Under high temperature and pressure, nucleation of bismuth tungstate is likely to take place, which may later grow into small nanoparticles. Under continued hydrothermal heating and a highly acidic environment, these aggregated nanoparticles may further grow into a two-dimensional structure followed by anisotropic growth, crystallization and a self-assembling process, eventually leading to the formation of a flower-like hierarchical structure. A mesoporous surface was obtained after removal of the organic surfactant.

Fig. 1 (A) Powder XRD and (B) FTIR spectra of mesoporous and non-mesoporous Bi₂WO₆.

Fig. 2 (A & B) FESEM, (C) HAAD-TEM and (D) EDS of Pt/mesoporous Bi₂WO₆.

Scheme 1 Schematic of nucleation, growth, crystallization and self-assembling process of mesoporous Bi₂WO₆.

The mesoporous structure of bismuth tungstate was confirmed by BET analysis and an exemplary nitrogen adsorption–desorption isotherm of this mesoporous sample is depicted in Fig. 3(A). Isotherms were found to be classical type IV, which is characteristic of mesoporous materials. The pore size distribution, shown as the inset, ranged between 2.5 and 5 nm and the average pore size distribution was around 3.5 nm. The average pore size distribution and BET surface area of mesoporous Bi₂WO₆ was investigated. The pore sizes were found to be 3.2, 3.3, 3.5 and 3.9 nm, whereas the BET surface areas were measured as 41.3, 38.1, 34.7 and 25.8 m² g⁻¹ for Bi₂WO₆ synthesized at 130, 150, 170 and 190 °C, respectively. The surface area of the non-mesoporous sample synthesized at 170 °C was found to be 24.6 m² g⁻¹. A decrease in the surface area with increasing synthesis temperature may be rationalized in terms of the greater extent of condensation of Bi₂WO₆ layers at higher temperature, which in turn leads to shrinkage or even destruction of the hierarchical structure, consequently resulting in a lower surface area. This assumption was substantiated by microscopic images which showed a partially destroyed hierarchy for the sample synthesized at 190 °C (Fig. S2†).

Fig. 3 (A) N₂ adsorption–desorption isotherms together with pore size distribution (inset figure) and (B) DRS of Pt/mesoporous Bi₂WO₆.

Optical properties of the bismuth tungstates were investigated by diffuse reflectance spectroscopy. No obvious difference in the absorption spectra of the various samples was found, and the absorption spectrum of mesoporous bismuth tungstate (prepared at 170 °C) is presented in Fig. 3(B). For crystalline and direct bandgap semiconductors, the absorption coefficient satisfies the equation (ahν)² = A(hν − E_g) for a, where a, ν, A and E_g are the absorption coefficient, light frequency, proportionality coefficient and bandgap energy, respectively.³⁶ The bandgap obtained by extrapolation of the plots (inset figure) of (ahν)² vs. hν was 2.8 eV, which corresponds well into the visible region.

All the photocatalytic experiments were carried out under visible light irradiation (>420 nm) using a longpass filter. Apart from the fact that RhB is a widely used dye in various industries, it has also been chosen as the model pollutant to study the photocatalytic performance of bismuth tungstate in many previous investigations.^20,21,23,29 Hence, RhB was selected as the model pollutant to study the photocatalytic activity so that the results of this study could be correlated to that of previous studies documented in the literature, as needed. Furthermore, noting the fact that dyes also absorb light, particularly visible light, and may lead to an elusive dye-sensitized photochemical process rather than a photocatalytic process, a test experiment was performed in which excitation of a bismuth tungstate aqueous suspension containing RhB was carried out at a wavelength >500 nm. The light intensity of >500 nm radiation was adjusted close to that of >420 nm radiation using neutral density filters. Analysis of the irradiated dye samples under >500 nm radiation did not show any noticeable change in RhB concentration, indicating that the photocatalytic performance evaluated in this study was truly photocatalytic.

Fig. 4(A) shows the change in relative concentration of RhB as a function of the irradiation time in the presence of bare mesoporous Bi₂WO₆, Pt/non-mesoporous Bi₂WO₆ and Pt/mesoporous Bi₂WO₆.

Fig. 4 (A) Comparative photocatalytic activity of Bi₂WO₆ samples and (B) temporal evolution of RhB absorption spectra in the presence of Pt/mesoporous Bi₂WO₆; (a) RhB, (b) N,N-diethyl-N′-ethyl rhodamine, (c) N,N-diethyl- or N-ethyl-N′-ethyl rhodamine, (d) N-ethyl rhodamine, and (e) rhodamine (Rh).³⁷

The degradation curves can be fitted reasonably well by an exponential decay curve suggesting first order kinetics. As shown, a remarkable increase in the photocatalytic efficiency of the mesoporous catalyst was obtained after Pt modification, and the Pt-modified mesoporous sample exhibited better photocatalytic efficiency compared with Pt/non-mesoporous Bi₂WO₆; more than 95% of RhB removal was achieved within 20 min in the presence of Pt/mesoporous Bi₂WO₆, while only ∼75% of dye was removed with the Pt/non-mesoporous sample. The better efficiency of the mesoporous photocatalyst can be explained in terms of increased surface area. As stated above, the surface area of the mesoporous and non-mesoporous samples were found to be 34.7 and 24.6 m² g⁻¹, respectively. The higher surface area and porous structure seem to facilitate the adsorption–desorption kinetics of RhB, as well as by-products formed during the photocatalytic process making the photocatalytic removal of dye more efficient. A blank experiment (without photocatalyst) was also carried out under visible light and analysis of the irradiated dye solution did not show any noticeable change in concentration, as delineated in Fig. 4(A). Interestingly, in the presence of Pt/Bi₂WO₆, a blue shift in the dye absorption spectra, together with a change in concentration, was noticeable with respect to the irradiation time. A typical evolution of RhB absorption spectra in the presence of Pt/mesoporous Bi₂WO₆ with respect to the irradiation time (30 min) is depicted in Fig. 4(B). Apparently, the absorption intensity of RhB decreased at 553 nm upon irradiation and a gradual wavelength shift occurred from higher to lower wavelength (from 553 to ∼500 nm). Under continued irradiation, the peak around 500 nm also disappeared, implying complete removal of dye. Such a characteristic disappearance of RhB was also observed with pure Bi₂WO₆ in our study, as well as in a study reported by Fu et al.,²⁹ which indicated that neither mesoporosity nor Pt seem to interfere in the degradation route of RhB but, nevertheless, enhance the overall photocatalytic removal of RhB. Since RhB is a tetra-ethylated organic dye, the hypsochromic transition could be correlated with a de-ethylation process followed by destruction of the chromophoric structure.³⁷

As stated above, the synthesis temperature may have a significant impact on various properties of Bi₂WO₆ which determine the photocatalytic property. The effect of various temperatures, such as 130, 150, 170, and 190 °C, on the photocatalytic activity was investigated and the obtained results are delineated in Fig. 5(A). It can be readily seen that the photocatalytic activity increases with the increase in synthesis temperature from 130 to 170 °C, presumably owing to the formation of a hierarchical structure and improvement in crystallization as indicated by microscopic and XRD analysis. The highest photocatalytic activity was obtained with the sample synthesized at 170 °C followed by a decrease at 190 °C. The higher photocatalytic activity seems to be due to an optimum compromise between the surface area, degree of crystallinity and hierarchical structure. On the other hand, the lower activity shown by the samples prepared at 130 °C and 150 °C could be due to a poor degree of crystallinity and a smaller fraction of hierarchy attained, which was not compensated by the positive effect of high surface area. Furthermore, the decrease in photocatalytic activity of the catalyst synthesized at 190 °C could be explained by the large decrease in surface area, together with the loss of hierarchical structure, as observed by BET and SEM analysis.

Fig. 5 (A) Effect of synthesis temperature and (B) Pt amount on the photocatalytic efficiency of Pt/mesoporous Bi₂WO₆.

Since Pt significantly triggered the photocatalytic efficiency of Bi₂WO₆, the amount of deposited Pt may play an important role in the optimization of the photocatalytic efficiency of the resulting bimetallic photocatalyst. To investigate the effect of the amount of Pt on the photocatalytic activity, Bi₂WO₆ synthesized at 170 °C was chosen as it showed the highest activity. Varying amounts of platinum (0.25, 0.5, 1.0, 1.5 and 2.0 wt%) were photodeposited onto the surface of mesoporous Bi₂WO₆ and the resulting photocatalytic activity was investigated. The maximum decomposition of RhB was obtained with 0.5 wt% Pt followed by a decrease at higher metal loadings, as illustrated in Fig. 5(B). The improvement in decomposition of RhB in the presence of platinized bismuth tungstate may be attributed to the possible formation of a Schottky barrier between Pt and Bi₂WO₆ and an enhanced surface area of Pt/Bi₂WO₆. In general, the formation of a Schottky barrier between noble metals and semiconductor photocatalysts has been discussed previously by other authors.^3,38,39

Briefly, the enhancement in photocatalytic activity may be ascribed to the imbalance between electron and hole densities, which is caused by the quicker oxidation of reductants by holes than oxidant reduction by electrons.³ This gives the potential gradient according to Poisson's equations as well as the carrier concentration gradient.⁴⁰ These two gradients, according to the hole and electron transport equations,⁴⁰ are the driving factor for charge carriers to drift and diffuse. When the platinum nanoparticles, having an electron withdrawing capability, are deposited on the Bi₂WO₆ surface, a Schottky barrier is likely to be formed at the interface of Bi₂WO₆ and Pt, which facilitates the channelling of electrons from the bulk of Bi₂WO₆ to the newly formed interface. As a result, the number of electrons in Bi₂WO₆ decreases, which in turn prevents the electron–hole pair recombination (or enhances the availability of holes for oxidation) and hence a higher photocatalytic activity was observed. A decrease in photocatalytic efficiency at higher Pt loadings may be rationalized by the fact that when the electron density gradient in the bulk of Bi₂WO₆ decreases substantially owing to the transportation of electrons, the electrical potential gradient decreases and finally the rate of electron diffusion decreases. When the two gradients are too small to further increase the electron flux through the Pt–Bi₂WO₆ Schottky barrier, a new equilibrium is attained and the additional deposition of platinum particles is unable to create further separation of electrons and holes.⁴¹ A fast electron relay, due to the higher amount of Pt nanoparticles (>0.5 wt% in our study), from Bi₂WO₆ to Pt can deform the potential field in Bi₂WO₆ particles and draw a part of the holes near the Pt–Bi₂WO₆ junction, which can facilitate the electron–hole pair recombination. The increase in the Pt–Bi₂WO₆ contact area can enhance the probability for the recombination of charge carriers and reduce the overall photocatalytic activity. Moreover, a higher amount of Pt can also work as a shield and prevent the incident photons from impinging on the Bi₂WO₆ surface, thereby decreasing the photocatalytic performance.⁴¹ Interestingly, in this study, it appears that the role of Pt is not limited to only being an electron sink, as discussed above, but also to render the interaction between RhB and the photocatalyst surface. This observation was substantiated by performing controlled experiments in the absence of light, and the change in RhB absorption spectra, or concentration with respect to time, is presented in Fig. S5.† Controlled experiments that were carried out in the dark indicated that ∼82% RhB was adsorbed on Pt/mesoporous Bi₂WO₆ as compared to ∼60% adsorption on pure mesoporous Bi₂WO₆ after 60 min. The concentration of RhB decreased monotonically with time and no shift in the absorption spectrum, unlike with the photocatalytic process, was observed indicating strong adsorption of RhB on the Pt/Bi₂WO₆ surface. The greater adsorption of RhB on Pt/Bi₂WO₆ could be correlated to the enhanced surface area; interestingly, the surface area of platinized bismuth tungstate was found to be 50.35 m² g⁻¹, which was ∼45% higher than pure mesoporous Bi₂WO₆. The enhancement in surface area may be attributed to the deposition of small (∼2 nm) Pt nanoparticles. The higher surface area rendered greater adsorption of RhB and better interfacial charge transfer between the catalyst's surface and RhB molecules, which ameliorated the removal of dye.

The photocatalytic activity of Pt/mesoporous Bi₂WO₆ was compared with other photocatalysts which are widely studied and considered to be highly active visible-light-driven photocatalysts, namely N-doped TiO₂, Pt/N-doped TiO₂ and Pt/WO₃ (ref. 6 and 42) and the obtained results are presented in Fig. 6. As shown, Pt/mesoporous Bi₂WO₆ showed much better activity than the other photocatalysts studied. For the first 20 min of irradiation, the photocatalytic removal values of dye in the presence of N-doped TiO₂, Pt/WO₃, Pt/N-doped TiO₂ and Pt/Bi₂WO₆ were found to be ∼34%, ∼58%, ∼72% and ∼96%, respectively. The superior photocatalytic performance of Pt/Bi₂WO₆ may be rationalized in terms of the following: (a) strong interaction between Pt/Bi₂WO₆ and RhB, as discussed above, and (b) the standard redox potential of Bi^V/Bi^III. The standard redox potential of Bi^V/Bi^III is more negative (+1.59 V) than the required potential (OH/OH⁻, +1.99 V) for water oxidation to generate OH radicals.⁴³ The theoretical assumption that Bi₂WO₆ does not have a sufficient redox potential to produce OH radicals was corroborated in earlier studies.^44,45 With this noted fact that Bi₂WO₆ is incapable of producing OH˙ through water oxidation, it may be strongly anticipated that the separated holes, in the presence of Pt, will be directly involved in the photooxidation of dye which will cause faster removal of RhB in the presence of Bi₂WO₆ compared with the other studied photocatalysts.

Fig. 6 Comparison of the photocatalytic activity of Pt/mesoporous Bi₂WO₆, N-doped TiO₂, Pt/WO₃ and Pt/N-doped TiO₂.

In addition to the study of the photocatalytic properties, the photoelectrochemical behavior of mesoporous Bi₂WO₆with and without Pt was also investigated. Noting the fact that oxygen is an efficient electron acceptor, experiments were also performed in the presence and absence of O₂. The potentiodynamic behavior of both the electrodes in the presence and absence of O₂ monitored under visible light is shown in Fig. 7(A). Since the electrodes may show a certain degree of current drift over time scales of 5–10 min and hence create ambiguity between the photocurrents under illumination and the dark current, photocurrents (under illumination and dark current) were measured in a single experiment by alternately turning the light on and off every 20 seconds for 15 min at a constant applied voltage of 0.95 V and the obtained results are presented in Fig. 7(B) and (C). The photocurrent was generated instantaneously upon illumination and reached almost steady state, while a negligible current was observed in the dark, even at a high applied potential (0.95 V). As illustrated in Fig. 7(B) and (C), both of the electrodes possessed similar and reasonably good stability under the studied experimental conditions. Interestingly, the photocurrent density of Bi₂WO₆ was found to be highly dependent on Pt and O₂. In the presence of O₂, the photocurrent density of both the electrodes, pure Bi₂WO₆ as well as Pt/Bi₂WO₆, remained almost intact and comparable. Interestingly, in the absence of oxygen, the photocurrent generated by the Pt/Bi₂WO₆ electrode was again comparable but in the case of pure Bi₂WO₆, a dramatic decline in the photocurrent density was observed. It seems reasonable to rationalize this phenomenon in terms of the fact that the generated electrons, in the case of Pt/Bi₂WO₆, were scavenged by Pt nanoparticles and O₂ appears to have a less obvious role to play as an electron acceptor on the photocatalyst’s surface. Hence, in the presence of Pt/Bi₂WO₆, the overall flow of photocurrent remained almost intact and irrespective of the presence or absence of oxygen. Moreover, in the absence of Pt or in the case of bare Bi₂WO₆, excited electrons seem to be taken up by oxygen, generating the analogous flow of photocurrent. However, when the analysis was performed in the absence of both oxygen and Pt, a remarkable decrease in the photocurrent density was observed presumably due to electron–hole pair recombination, which is extremely efficient in the absence of electron acceptors or donors. These findings advocated that both of the entities, Pt as well as O₂, serve as an effective electron acceptor.

Fig. 7 (A) Potentiodynamic behavior and (B & C) voltammograms under intermittent illumination/darkness of Bi₂WO₆ and Pt/mesoporous Bi₂WO₆ in the presence and absence of O₂.

According to the photoelectrochemical observations, which showed analogous generation of charge carriers in pure and Pt/Bi₂WO₆ in the presence of O₂, a comparable photocatalytic activity of bare and Pt/Bi₂WO₆ in the presence of O₂ may be anticipated. However, in the presence of O₂, the photocatalytic activity of Pt/Bi₂WO₆ was much better compared with pure Bi₂WO₆. This anomaly may possibly be ascribed to effective interaction of RhB dye with the surface of the platinized bismuth tungstate, as discussed above, and to valence band holes (as effective oxidants). Furthermore, in order to elucidate the role of O₂ in the photocatalytic decomposition of RhB, photocatalytic experiments were carried out in the presence or absence of oxygen and the results are illustrated in Fig. 8. The removal of RhB, in the case of both pure as well as Pt/mesoporous Bi₂WO₆, was faster in the presence of O₂ revealing the involvement of O₂ in the photooxidation of RhB as an oxidant. In the case of bare Bi₂WO₆, O₂ may take excited electrons from the photocatalyst's surface to generate reactive O₂⁻˙ and more importantly make the valence band holes available for the photooxidation of dye. As a result, significant improvement in the photocatalytic efficiency of Bi₂WO₆ was observed. Interestingly, in the case of Pt/Bi₂WO₆, although the photocatalytic response was improved in the presence of O₂, it was not as notable as in the case of bare Bi₂WO₆. So, despite the fact that O₂ played a less significant role in the case of Pt/Bi₂WO₆, removal of RhB was significantly enhanced. This observation indicated the crucial involvement of valence band holes in the photocatalytic oxidation of RhB. As discussed above, Pt-modified Bi₂WO₆ has a higher surface area, which rendered greater adsorption of RhB and better interfacial electron transfer from RhB to the valence band holes.

Fig. 8 Effect of O₂ on the photocatalytic performance of mesoporous Bi₂WO₆ samples; (a) Pt/Bi₂WO₆ (with O₂), (b) Pt/Bi₂WO₆, (c) Bi₂WO₆ (with O₂), and (d) Bi₂WO₆.

So, based on the collective photocatalytic and photoelectrochemical observations, a plausible mechanism involving electrons, holes, Pt and RhB dye has been proposed in Fig. 9. Briefly, the excited conduction band electrons can be picked up by Pt, or by O₂ in absence of Pt, and O₂ can be reduced to O₂⁻˙. These O₂⁻˙ radicals can readily attack the adsorbed RhB and degrade it. On the other hand, valence band holes can directly oxidize the RhB, which was observed to be the dominant route, and destroy the chromophoric structure of RhB via the formation of different intermediate products as discussed above.

Fig. 9 Schematic showing the involvement of Pt, O₂, electrons and holes in the degradation of RhB dye.

Conclusions

Mesoporous hierarchical bismuth tungstate was synthesized by a hydrothermal method using Pluronic F127 followed by surface modification with Pt nanoparticles. The hierarchical structure, mesoporosity, surface area and photocatalytic activity were found to be dependent on the synthesis temperature. Surface modification with Pt nanoparticles dramatically augmented the photocatalytic activity of mesoporous Bi₂WO₆ and the resulting activity was better than with Pt/WO₃, N-doped titania and Pt/N-doped titania. Photoelectrochemical data revealed the analogous generation of excited charge carriers in Bi₂WO₆ in the presence of Pt or O₂, but the photocurrent density was substantially diminished in the absence of electron acceptors such as Pt or O₂. Based on the photocatalytic and photoelectrochemical findings, it was deduced that Pt deposited on Bi₂WO₆ played a dual role: (1) as electron acceptor and (2) as additional sites for RhB dye to be adsorbed. Furthermore, valence band holes seemed to have a crucial contribution as oxidant in dye removal, in addition to O₂.

Acknowledgements

The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project no. 10-NAN1387-04 as part of the National Science, Technology and Innovation Plan. The support of Center of Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45948a

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